1. Field of the Invention
The present invention relates generally to ultrasound imaging and more particularly to speckle reduction in ultrasound imaging.
2. Description of the Related Art
Ultrasonic imaging has become one of the most important and popular diagnostic tools, with a wide range of applications. Particularly due to its non-invasive and non-destructive nature, ultrasound imaging has been extensively used by the medical profession.
One fundamental problem in all types of imaging is noise from backscattered signals, which obscures the details of the target image or echo. One type of noise, commonly known as "speckle" in tissue characterization and optics, results from, constructive and destructive interference, and appears as a random mottle superimposed on the image. Normally, speckle is from objects whose dimensions are smaller than the wavelengths of the radiation source, making it impossible to eliminate the speckle simply by increasing the resolution of the imaging device. Moreover, speckle is from objects that are stationary and randomly distributed. Since the speckle has no phase or amplitude variation as a function of time, one cannot suppress it by averaging the imaging signals in time. In other words, speckle signals are coherent, and cannot be reduced through averaging.
One method to reduce speckle is through a method known as spatial diversity or spatial compounding. The idea is to capture the target image a number of times with the radiation illuminating the target from different directions. The multiple images are then combined to remove the speckle. The success of the method is a result of the statistical independence of speckle patterns, and the fact that the target size is much larger than the speckle-causing scatterers. By taking images from a number of directions, the speckle is made to behave like uncorrelated time-varying noise, while the target echoes remain correlated and virtually unchanged. With a number of those images combined together through spatial compounding, speckle can be reduced. The calculations to combine images formed from different directions for reducing speckle are well known.
Typically, the way to generate multiple images from different directions is to excite different sections of a linear array of piezo-electric sources/detectors or transducer elements, which generate and measure the ultrasound. For example, one can separate a linear array of 128 transducer elements into 4 successive sections, each section with 32 elements. The sections are excited one at a time, with the ultrasound beam from each section steered so that all four beams are focused at substantially the same region, but from different directions. Typically, steering of a beam is done through controlling the delays of the signals from a number of transducer elements; such techniques are well-known to those skilled in the art. Speckle can then be reduced by combining the four echoes from the four different directions. The problem with this method is related to the aperture size of the array, which is defined as the area where the transducer elements are activated to generate the beam. Instead of using the entire array, one can only use a part of the array at one time, which significantly reduces the aperture size of the array, and the lateral resolution of the images formed. Also the reduced aperture size would significantly reduce the signal strength for imaging and decrease the signal-to-noise ratio of the images formed.
A paper, titled, "Speckle Reduction in Pulse-Echo Ultrasonic Imaging Using a Two-Dimensional Receiving Array," written by Giesey et al., and published in IEEE Transaction on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 39, No. 2, March 1992, describes a two-dimensional transducer to reduce speckle. In that paper, the transducer includes a center disk of transducer elements, which are surrounded by eight concentric annular arrays or annuli of transducer elements, with each annulus having a different radius. Each annulus is divided into eight equal segments, creating a total of sixty-four separate segments. In operation, the center disk would generate an ultrasound pulse, and then different segments would capture the echo from the pulse to generate an image of the region interrogated. With different segments capturing the echo from different directions, Giesey can again reduce the speckle in the image. However, similar to the one-dimensional array, Giesey has to trade-off lateral resolution or aperture size--using segments--to reduce speckle.
From the above discussion, it should be apparent that there is a need to reduce speckle Without requiring significant trade-offs. In other words, one would like to reduce speckle, without significantly reducing the aperture size of the transducer, or the lateral resolution of the images formed.